WASHINGTON -- In exposing America's vulnerability, terrorists opened up what promises to be a contentious new struggle over American national security.

Apparently armed only with knives, brawn and brains, terrorists melded two time-tested tactics -- hijacking and suicide attacks -- to convert three jetliners into airborne truck bombs.

That such crude means could hoodwink U.S. intelligence, airport security and air defenses to wreak colossal destruction has shaken up the national security establishment and sent the war on terrorism to the top of American foreign policy priorities.

As a result, experts expect the United States to pump more money into defense and intelligence, but the debate is sure to grow over how to spend it. And as counterterrorism dominates America's global agenda, it will cause a ripple effect through a host of alliances and relationships worldwide.

Part of the problem in combating terrorists is that their possible modes of attack are almost limitless.

"We can come up with another 100 horrible things that could be true -- and we'd be wrong on almost all of them," said Daniel Blyman, research director at the RAND Corp.'s Middle East center.

After members of a Japanese cult sent poison gas wafting through the Tokyo subway system in 1995, the threat of chemical and biological attacks became the "flavor of the day" in counterterrorism, he said.

Advocates -- whether the Bush administration or members of Congress -- might try to sell many new spending proposals as essential to the war on terrorism.

In this environment, President Bush's missile defense plan -- opposed by many Democrats -- faces an uncertain fate.

The president could try to rally support for his plan by exploiting the nation's new feeling of being unprotected against an array of potential foreign threats.

"It could put Democrats on the defensive, showing that they are not willing to 'bear any burden,'" said Janne E. Nolan, international director of the Eisenhower Institute for Global Affairs, a Washington think tank. "People won't think through whether this threat is appropriate to missile defense, but will rather focus on anything to reduce vulnerability."

But critics are likely to try to undercut the president's case for spending billions on missile defense, arguing with renewed vigor that -- as Tuesday's events demonstrated -- even an impregnable missile shield can't prevent terrorists from slipping into the country and killing thousands of people.

There's another argument that opponents may use: Bush has threatened to pull out of the 1972 Anti-Ballistic Missile Treaty to avoid being restricted in developing a missile defense. But critics say that could lead to a lessening of international support for any U.S.-led efforts to limit the spread of weapons of mass destruction.

Such efforts are especially needed now to prevent terrorists from getting their hands on such weapons, says Lynn E. Davis, a former undersecretary of state and a senior fellow at RAND. For terrorists to acquire such weapons might be difficult, "but what we should have learned is that if there's a will, there's a way," she said.

The new climate also promises to change American relationships overseas.

The United States won support from its NATO allies yesterday in condemning Tuesday's attacks. But some allies say the Bush administration shows a tendency to "go it alone" in foreign policy, even though Secretary of State Colin L. Powell said yesterday that the United States wants to build a global coalition against terrorism.

"We can't do it ourselves, even if we quadruple the defense budget," said Sam Nunn, who as U.S. senator headed the Armed Services Committee and now co-chairs the Nuclear Threat Initiative, a private foundation dedicated to reducing the global threat posed by weapons of mass destruction. He said the United States should reach out to gain Russia's cooperation.

If and when the Bush administration decides to retaliate against the group or state behind Tuesday's attacks, it should "let our friends know what we're doing," Nunn said. Allies in the Arab world "could be subject to dislocation if we blindside them."

The impending war on terrorism also could blind the United States to the faults of countries whose support it needs. For instance, the United States might conclude that it needs to enlist Pakistan in gaining information on the Taliban in Afghanistan, which has been harboring suspected terrorist mastermind Osama bin Laden. To win that support, the United States might have to overlook the collapse of democracy in Pakistan or Islamabad's backing of rebels in disputed Kashmir.

"If you think about the Cold War, we tolerated authoritarian governments because the alpha and omega of our foreign policy was the anti-Communist alliance. There were all kinds of trade-offs," said James Steinberg, director of foreign policy studies at the Brookings Institution.